Cavity enhanced lab-on-fiber optrode for ultra-sensitive pH monitoring

Abstract

Current technologies for pH monitoring still require the use of large and rigid tools that are not suitable for studying biological processes such as cell culture and tissue metabolism analysis. In this work, we report on a miniaturized cavity enhanced optrode based on lab-on-fiber technology for pH monitoring in liquid solutions. The device consists of a resonant cavity directly integrated on top of a single mode optical fiber, where the active medium is made of a responsive material (microgel) suitably synthesized to respond to pH variations. The pH variations in the liquid solution in which the probe is immersed modulate the optical cavity length, inducing a wavelength shift of the interference fringes in the reflection spectra. Remarkable sensitivities of up to 315 nm per pH unit are experimentally observed in the pH range from 4.6 to 5.8. Moreover, sensitivities >20 nm per pH unit are found in a wide pH range from 2.9 to 8. To the best of our knowledge, these represent the highest values reported so far with optical fiber based pH sensors. Our findings set the stage for the development of compact, flexible, and highly sensitive pH sensors, opening unexplored scenarios also for in vivo pH monitoring.